The present invention is directed toward wireless networks and more particularly to systems and methods for sharing access to a common wireless transmission medium.
In a wireless network with multiple data communication devices, an important problem to solve is how to schedule access to a shared wireless transmission medium. Typically, spectrum is limited. Except for spread spectrum systems which present other management challenges such as power control, only one device can be heard at a time on a given frequency. One technique for organizing access to the common wireless transmission medium is known as CSMA (Carrier Sense Multiple Access) with a related technique being known as CSMA-CD (Carrier Sense Multiple Access—Collision Detection). In these techniques, a device having data to communicate listens for other devices. If no other transmissions are heard, the device having data to communicate begins its own transmission. In the CSMA-CD variant, the device is capable of sensing that another device has begun transmitting simultaneously. This is known as a collision and both devices stop transmitting and then reattempt transmission after pseudo-randomly chosen delay times.
Unfortunately, a limitation of this technique is that not every data communication device in the network is within communication range of all the others. Consider a situation where a device B can communicate directly with both A and C but A and C are separated by too large a distance to detect when the other is transmitting, and are therefore unable to communicate directly. To illustrate the problem that can arise, suppose A is transmitting to B. Since C cannot detect A's transmissions, it will mistakenly assume that the medium is not being used. Then suppose that C, mistakenly believing that the bus is idle, attempts to transmit a message to B. As a result, a data collision occurs at B and the messages transmitted by A and C are both corrupted or one of the messages is lost. A situation such as this is commonly referred to as the “hidden transmitter problem.” When the wireless communication network is implementing higher level protocols such as IP and TCP, a result of the hidden transmitter problem is that the higher level protocols must frequently retransmit data and the physical layer is therefore used very inefficiently.
An alternative technique that solves the hidden transmitter problem involves providing a specialized data communication device known as a central access point. The central access point is able to communicate with all of the other nodes of the wireless communication network. The central access point distributes a transmission schedule to the other nodes and this transmission schedule is followed, theoretically eliminating the possibilities of collisions. The network nodes other than the central access point are then referred to as “subscriber units.”
In this architecture, however, all of the subscriber unit nodes must be within range of the central access point. Even if communication is theoretically possible because a remote node can reach one of the subscriber units of the network, the remote node cannot be accommodated if it is out of range of the central access point.
Another solution is a split frequency system. All of the nodes transmit on frequency F1 and receive on frequency F2. A central rebroadcast node rebroadcasts everything heard on F1 on F2. This provides a mechanism for a CSMA or CSMA-CD system to ensure that all nodes can hear one another. Again there must be a central device capable of direct communication with all other nodes. What is needed is a wireless medium access control architecture that does not require frequent retransmissions due to collisions, does not require that a central node be within range of all other nodes, and that can adapt easily to addition of new network nodes.